Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (NO) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization-immobilization and NO emission. Residues were added at the rate of 3 t C ha to soil with, and without, 150 kg urea N ha. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (Ṕ 0.05) lower NO -N, which reached minimum values of 2.8 mg N kg for sugarcane (at day 28), 10.3 mg N kg for maize (day 7), and 5.9 mg N kg for sorghum (day 7), compared to 22.7 mg N kg for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg in sugarcane, 34 mg kg in maize, 29 mg kg in sorghum, and 16 mg kg in cotton amended soil compared to soil + N fertilizer, although soil NO -N increased by 7 mg kg in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of NO from crop residue amended soils occurred in the first 4-5 days of incubation. Overall, after 84 days of incubation, the cumulative NO emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative NO emission was significantly and positively correlated with NO -N (r = 0.92, Ṕ 0.01) and total mineral N (r = 0.93, Ṕ 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO in the first 3 and 5 days of incubation (r = 0.59, Ṕ 0.05)